1. Field of The Invention The present invention relates to an ignition device for an engine. More particularly, it relates to an engine ignition device having an overspeed proof function wherein an engine speed is detected.
2. Discussion of Background
FIG. 2 is a block diagram showing a construction of conventional engine ignition device having an overspeed proof or governor function. In FIG. 2, a reference numeral 1 designates a signal generator (SG), a numeral 2 designates a waveform shaping circuit which has an input terminal connected to the terminal of positive pole of the signal generator 1, and a numeral 3 designates a closing rate controlling circuit having an input terminal connected to the positive side of the signal generator 1 and another input terminal connected to a current detection circuit 9 which will be described hereinafter. A reference numeral 4 designates a bias circuit having a terminal connected to the closing rate controlling circuit 3 and another terminal connected to the negative side of the signal generator 1. A numeral 5 designates a capacitor which has an end connected to the closing rate controlling circuit 3 and another end connected to the ground. A numeral 6 designates a logical operation circuit which has an input terminal connected to the waveform shaping circuit 2 and a comparing circuit 10c which will be described later. A numeral 7 designates an overvoltage protective circuit having an input terminal connected to an ignition coil 12 and a battery (not shown). A numeral 8 designates an amplifying circuit having input terminals connected respectively to the logical operation circuit 6, the overvoltage protective circuit 7 and a current detection circuit 9. The current detection circuit 9 is connected to a resistor 14 the opposite end of which is grounded. A numeral 10 designates an ignition-cut controlling circuit which comprises a frequency-voltage transducing circuit 10a connected to a capacitor 10b the opposite pole of which is grounded and a comparing circuit 10c connected to an output terminal of the frequency-voltage transducing circuit 10a. An input terminal of the ignition-cut controlling circuit 10 is connected to the output terminal of the waveform shaping circuit 2. A numeral 11 designates a power transistor which has its base connected to the amplifying circuit 8, its collector connected to the ignition coil 12 and its emitter connected to the ground through a resistor 14 respectively. A numeral 13 designates an ignition plug connected to the secondary winding side of the ignition coil 12 which is, in turn, connected to a battery (not shown).
In the conventional engine ignition device having the construction described above, a bipolar, output is produced at the signal generator 1 to be supplied to the waveform shaping circuit 2 where the output signal is subjected to waveform shaping, whereby a pulse signal having a predetermined time width is obtainable. The pulse signal having the shaped waveform is supplied to the input terminals of the frequency-voltage circuit 10a and the logical operation circuit 6. The pulse signal is subjected to a frequency-voltage transformation in the circuit 10a. A voltage transformed therein is compared with a predetermined value in the comparing circuit 10c. When the value of voltage compared is larger than the predetermined value, the comparing circuit 10c generates an output signal which is supplied to the logical operation circuit 6. The logical operation circuit 6 is constituted in such a manner that when it receives an output from the comparing circuit 10c, it does not generate an output. Namely, when a voltage transformed from a frequency is lower than the predetermined value, an output is produced from the logical operation circuit 6. The output signal of the logical operation circuit 6 actuates the power transistor 11 through the amplifying circuit 8, whereby a voltage is produced at the primary winding of the ignition coil 12, hence a high voltage is induced at the secondary winding of the ignition coil 12 to thereby ignite the ignition plug 13. On the other hand, when there is no output from the logical operation circuit 6, the power transistor 11 is not actuated, hence no voltage is produced at the primary winding of the ignition coil 12, and an operation of ignition does not take place. Namely, when the revolution speed of the engine exceeds a predetermined revolution, the ignition-cut controlling circuit 10 operates to stop the ignition whereby the overspeed operation of the engine is avoidable.
In the conventional engine ignition device, a tachometer is connected to the primary winding side of the ignition coil 12 so that an engine speed is detected. When the engine becomes a state of overspeed, a voltage to the primary winding of the ignition coil 12 is cut and therefore, it is impossible to detect the engine speed after the cutting.
In the conventional engine ignition device wherein the engine speed is detected by detecting the primary voltage produced at the primary side of the ignition coil 12, the engine speed of the engine cannot be correctly detected.